In microwave digital communication, a unit (IDU) including a processing section and a modem section for baseband signals and a unit (ODU) including a high frequency circuit are placed in an indoor location and an outdoor location respectively, and these units are connected by a cable, thus serving as a communication apparatus.
In this configuration, the cable connecting the IDU and ODU becomes a cause of signal degradation. Signal degradation due to the cable (signal degradation due to the fact that the cable is a transmission medium) is composed of practically uniform signal level reduction over entire bandwidth and signal level reduction which increases as the frequency becomes higher (linear slope).
The longer the length of the cable between the IDU and ODU, the amount of the practically uniform signal level reduction over entire bandwidth increases and the inclination of the linear slope of frequency characteristic becomes larger. Therefore, the longer the length of the cable between the IDU and ODU, the more a high frequency signal is degraded.
Since a communication device at the receiving side can compensate signal degradation due to the cable by a demodulator, communication itself is allowed even if compensation is not performed by a device at the sending side. However, in practice, there has been defined a standard of radio signals transmitted from the communication device at the sending side, and therefore, some compensation is required also at the communication device at the sending side for sending a radio signal in conformity with the standard.
As a conventional art for compensating signal degradation due to a cable in a communication device at the sending side, “DIGITAL FPU TRANSMITTER AND DIGITAL FPU RECEIVER” has been disclosed in Patent Document 1.
[Patent Document 1] Japanese Patent Laid-Open No. 2002-280910